The invention relates to a clamp-on ultrasonic flowmeter, a flow rate-measuring structure, and a ultrasonic transmitting-receiving device.
The clamp-on ultrasonic flowmeter is attached to a outer surface of a pipe in which a fluid flows, for measuring from outside of the pipe a volume of the fluid flowing inside of the pipe. The clamp-on ultrasonic flowmeters are generally classified into two types. One utilizes a difference of propagating rates, and another utilizes the Doppler effect.
In the mode utilizing a difference of propagating rates, a pair of ultrasonic waves are propagated under such condition that one ultrasonic wave is propagated downstream to cross the stream of fluid while another ultrasonic wave is propagated upstream to cross the stream of fluid. Then, the time required for propagating the downstream ultrasonic wave between the predetermined distance and the time required for propagating the upstream ultrasonic wave between the same distance are compared to determine the flow rate.
In the mode utilizing the Doppler effect, the flow rate is determined by measuring a rate of particle or babble flowing with the fluid, under assumption that the particle or babble moves at a rate equal to that of the moving fluid. The moving rate of the particle or babble can be determined, by detecting variation of ultrasonic frequency from that of ultrasonic wave applied to the moving particle or babble to that of ultrasonic wave reflected to the moving particle or babble.
A representative constitution of a known clamp-on ultrasonic flowmeter is illustrated in FIG. 17 in the form of a sectional view. The clamp-on ultrasonic flowmeter of FIG. 17 utilizes a difference of propagating rates of ultrasonic wave. The clamp-on ultrasonic flowmeter is composed of a pair of ultrasonic transmitting-receiving devices 1a, 1b. The ultrasonic transmitting-receiving device 1a is composed of a ultrasonic transducer 2a and a ultrasonic propagating element in the form of wedge 3a. The ultrasonic propagating element 3a has a bottom surface 4a and a slanting surface 5a extending from one edge of the bottom surface 4a at an acute angle. The ultrasonic transducer 2a is attached on the slanting surface 5a. The ultrasonic transducer 2a has an electrode (not shown) and a lead line (not shown) on the side facing the propagating element 3a and on another side. The combination of the electrode and lead line serves to apply electric voltage to the transducer 2a. In the same way, the ultrasonic transmitting-receiving device 1b is composed of a ultrasonic propagating element 3b having a slanting surface 5b on which the ultrasonic transducer 2b is attached.
Each of the ultrasonic transducers 2a, 2b transmits ultrasonic wave to the ultrasonic propagating element when an electric voltage is applied thereto, while it produces an electric voltage when it receives ultrasonic wave. Accordingly, the ultrasonic transmitting-receiving device 1a, 1b equipped with a ultrasonic transducer functions as a transmitter and a receiver. The ultrasonic transmitting-receiving devices 1a, 1b are provided on a pipe 6 in such manner that the ultrasonic waves transmitted by the devices 1a, 1b propagate across the fluid 7 which flows inside of the pipe in the direction indicated by arrow 8, that is, on the route 9 (indicated by a dotted line) in the directions indicated in FIG. 17 by arrows 9a, 9b. 
The flow rate of the fluid 7 flowing inside of the pipe 6 is determined by the following method. First, a voltage pulse is applied to the ultrasonic transducer 2a of the ultrasonic transmitting-receiving device 1a, so as to transmit a ultrasonic wave. The ultrasonic wave propagates in the ultrasonic propagating element 3a, a wall of pipe 6, fluid 7, a wall of pipe 6 on the opposite side, and ultrasonic propagating element 3b on the route indicated in FIG. 17 by the dotted line 9. Subsequently, the ultrasonic wave is received by the ultrasonic transducer 2b of the ultrasonic transmitting-receiving device 1b, to output a voltage signal. A period of time (T1) from the time when the ultrasonic wave is transmitted by the ultrasonic transmitting-receiving device 1a to the time when the ultrasonic wave is received by the ultrasonic transmitting-receiving device 1b is detected. Subsequently, a voltage pulse is applied to the ultrasonic transducer 2b of the ultrasonic transmitting-receiving device 1b, so as to transmit a ultrasonic wave. The ultrasonic wave is then propagate on the same route, but in the opposite direction, and the ultrasonic transducer 2a of the ultrasonic transmitting-receiving device 1a receives the propagated ultrasonic wave. A period of time (T2) from the time when the ultrasonic wave is transmitted by the ultrasonic transmitting-receiving device 1b to the time when the ultrasonic wave is received by the ultrasonic transmitting-receiving device 1a is detected.
The period of time (T1) required for the propagation of ultrasonic wave from the device 1a to the device 1b along the arrow 9a differs from the period of time (T2) required for the propagation of ultrasonic wave from the device 1b to the device 1a along the arrow 9b. The period of time (T1) is shorter than a period of time required for propagating ultrasonic wave in still water because the ultrasonic wave from the device 1a to the device 1b is propagated at an increased rate by the aid of the flowing fluid, while the period of time (T2) is longer than a period of time required for propagating ultrasonic wave in still water because the ultrasonic wave is propagated from the device 1b to the device 1a against the stream of the fluid. Thus, the difference of the propagation period (T2xe2x88x92T1) is relative to the rate of movement of the flowing fluid 7. Therefore, the rate of movement of the flowing fluid is calculated from the difference of propagation period. The flow rate of the fluid 7 is then determined from the difference of propagation period and the sectional area of the inside of the pipe 6.
In the conventional clamp-on ultrasonic flowmeter, the ultrasonic propagating element in the wedge form is made of solid material such as epoxy resin, acryl resin, or stainless steal.
Thus, the clamp-on ultrasonic flowmeter is advantageous in that it can determine the flow rate with no direct contact with the flowing fluid. In order to employ the clamp-on ultrasonic flowmeter more advantageously, however, a study should be made on the clamp-on ultrasonic flowmeter for increasing the measuring sensitivity.
As is apparent from the principle of measurement adopted by the clamp-on ultrasonic flowmeter, if the ultrasonic wave impinges on the outer surface of the pipe, the measurement of flow rate cannot be made because no difference is produced between the periods of ultrasonic wave propagation. Accordingly, the ultrasonic wave should be applied on the pipe surface obliquely, namely, at an acute angle. However, if the ultrasonic wave is applied on the pipe surface obliquely, a portion of the ultrasonic wave is caused not to penetrate into the pipe wall. The decrease of penetration of ultrasonic wave on the outer surface of the pipe causes to reduce the sensitivity of the clamp-on ultrasonic flowmeter.
The decrease of penetration of ultrasonic wave occurring when the ultrasonic wave is applied on the pipe surface obliquely is caused by reflection of the obliquely applied ultrasonic wave on the pipe surface. The reflection increases when the ultrasonic wave is applied at a larger angle of incidence, and further when the difference of sonic propagation rate between the material of the ultrasonic propagating element and the material of the pipe.
If the difference of sonic propagation rate between the ultrasonic propagating element and the pipe decreases, the reflection of ultrasonic wave on the pipe surface decreases. Therefore, it may be assumed that the reflection decreases if the ultrasonic propagating element and the pipe are made of the same material.
However, this can not be done, because most of the materials for the production of pipes attenuate ultrasonic wave passing through them. The attenuation of ultrasonic wave is understood to be caused by conversion of the ultrasonic wave (longitudinal wave) into a traverse wave or conversion into thermal energy.
Examples of the conventionally employed pipe materials are stainless steel (rate of sonic propagation: approx. 5,000 m/sec.), vinyl chloride resin (approx. 2,200 m/sec.) and fluororesin (approx. 1,200 m/sec.).
Generally, it is specifically difficult to design a clamp-on ultrasonic flowmeter of high sensitivity for the use in conjunction with a pipe made of material having a small rate of sonic propagation (typically fluororesin).
Accordingly, the present invention has an object to provide a clamp-on ultrasonic flowmeter giving an improved high sensitivity.
The invention has another object to provide a flow rate-measuring structure giving an improved high sensitivity.
The invention has a further object to provide a new ultrasonic transmitting-receiving device.
The present inventor has succeeded in providing a clamp-on ultrasonic flowmeter in which the ultrasonic wave transmitted by the ultrasonic transducer efficiently penetrates into the pipe surface and accordingly which shows an improved high sensitivity, by constituting the ultrasonic propagating area of the ultrasonic transmitting-receiving device from a combination of an obliquely arranged ultrasonic propagating element which propagates ultrasonic wave transmitted by the ultrasonic transducer predominantly in the direction perpendicular to a plane of the transducer, and a ultrasonic propagating layer placed between the ultrasonic propagating element and the pipe, which has a specific viscosity or rate of penetration of need and a specific rate of sonic propagation as compared with a rate of sonic propagation of material of the pipe.
The invention resides in a flow rate-measuring structure comprising a pipe in which a fluid flows and a pair of ultrasonic transmitting-receiving devices arranged on the pipe at an outer surface thereof, each transmitting-receiving device comprising a composite of a ultrasonic transducer and a ultrasonic propagating element which propagates ultrasonic wave transmitted by the transducer predominantly in the direction perpendicular to a plane of the transducer, the composite being arranged at an acute angle from the center line of the pipe, and a ultrasonic propagating layer placed between the ultrasonic propagating element and the pipe, which has a viscosity in the range of 0.5xc3x9710xe2x88x923 to 3xc3x9710 Pa.sec at 25xc2x0 C. and a rate of sonic propagation in terms of V1 at 25xc2x0 C. satisfying the condition of 0.5 less than V1/V2 less than 1.7 in which V2 represents a rate of sonic propagation of material of the pipe at 25xc2x0 C.
The invention also resides in a flow rate-measuring structure comprising a pipe in which a fluid flows and a pair of ultrasonic transmitting-receiving devices arranged on the pipe at an outer surface thereof, each transmitting-receiving device comprising a composite of a ultrasonic transducer and a ultrasonic propagating element which propagates ultrasonic wave transmitted by the transducer predominantly in the direction perpendicular to a plane of the transducer, the composite being arranged at an acute angle from the center line of the pipe, and a ultrasonic propagating layer placed between the ultrasonic propagating element and the pipe, which has a rate of penetration of needle in the range of 10 to 300 at 25xc2x0 C. and a rate of sonic propagation in terms of V1 at 25xc2x0 C. satisfying the condition of 0.5 less than V1/V2 less than 1.7 in which V2 represents a rate of sonic propagation of material of the pipe at 25xc2x0 C.
The rate of penetration of needle is a value measured by the method defined in JIS K 2220 (2001). The weight applied in the penetration measurement is 9.38 g.
The invention further resides in a ultrasonic transmitting-receiving device comprising a composite of a ultrasonic transducer and a ultrasonic propagating element which propagates ultrasonic wave transmitted by the transducer predominantly in the direction perpendicular to a plane of the transducer, the composite being arranged at an acute angle from the center line of the pipe, and a ultrasonic propagating layer placed between the ultrasonic propagating element and the pipe, which has a viscosity in the range of 0.5xc3x9710xe2x88x923 to 3xc3x9710 Pa.sec at 25xc2x0 C. and a rate of sonic propagation at 25xc2x0 C. is in the range of 700 to 2,000 m/sec.
invention furthermore resides in a ultrasonic transmitting-receiving device comprising a composite of a ultrasonic transducer and a ultrasonic propagating element which propagates ultrasonic wave transmitted by the transducer predominantly in the direction perpendicular to a plane of the transducer, the composite being arranged at an acute angle from the center line of the pipe, and a ultrasonic propagating layer placed between the ultrasonic propagating element and the pipe, which has a rate of penetration of needle in the range of 10 to 300 at 25xc2x0 C. and a rate of sonic propagation at 25xc2x0 C. is in the range of 700 to 2,000 m/sec.
In the invention, the following embodiments are preferred.
(1) The ultrasonic propagating layer comprises a liquid.
(2) The ultrasonic propagating layer comprises water, oil, glycerol, water glass, vaseline, or grease.
(3) The ultrasonic propagating layer comprises elastic material.
(4) The ultrasonic propagating layer comprises polymeric gel.
(5) The ultrasonic propagating layer comprises polyurethane gel or silicone gel.
(6) The ultrasonic propagating layer is placed in a case having an opening on a bottom surface thereof.
(7) Both of the composite and the ultrasonic propagating layer are placed in a case having an opening on a bottom surface thereof.
(8) The ultrasonic propagating element is composed of a plurality of sheet units in which each sheet unit is composed of plural high modulus fibers aligned in parallel in resinous material.
(9) The sheet units of the ultrasonic propagating element are produced under the condition that the high modulus fibers in one sheet unit are arranged to make a right angle with the high modulus fibers in an adjoining sheet unit.
(10) The pipe is made of fluororesin.